The inkjet recording method is currently achieving rapid development in various fields, because the high-definition image can be recordable by relatively simple apparatus. The inkjet recording method is applied to various fields and the recording medium or the inkjet inks are used just for each purpose. Recently, the recording speed is extremely increasing and the development of printer which has performances applicable for the short run printing.
Basically, in the inkjet recording method, low viscosity is required for the inkjet ink, because fine droplets of inkjet ink have to be ejected stably from inkjet head and the image is formed on the recording medium by depositing these droplets. However color mixing phenomena so called bleed tends to occur by depositing plural of inkjet inks having low viscosity on the recording medium. On the contrary, to use inkjet inks having high viscosity so as to prevent bleed presents a dilemma of unstable jetting from an inkjet head.
Several methods are proposed so as to solve this dilemma. For example, the image forming method using hot-melt inks is well known. In this image forming method using hot-melt inks, bleed is prevented by jetting the ink melted by heating from inkjet head and by solidifying ink instantaneously by cooling just after depositing on the recording medium. However this method still has problems such as abrasion or stickiness, when the ink is jetting on the recording medium such as film which cannot absorb ink inside.
Other candidate method is a method using photo-curable inkjet inks. In this method using photo-curable inkjet ink, bleed is prevented by containing photo-curable monomer and photo initiator in the ink and by solidifying ink by irradiating after deposition on the recording medium. However this method still has problem of insufficient bleed prevention, because it is difficult to irradiate light at the same time as ink deposition on the recording medium and time-lag remains between ink deposition on the recording medium and the light irradiation.
One method to solve problems in the above methods, a hot-melt type photo-curable inkjet ink is proposed which utilizes both advantages of a hot-melt ink and a photo-curable ink.
For examples, proposed is a method that utilizes an ink having a compound which can be gelled at low temperature, and the ink is ejected from an inkjet head under high temperature, is turned into a gel by cooling after deposition of the recording medium, and is cured by light irradiation (for example, Patent documents 1 and 2). A further similar method is proposed which utilizes an ink having thixotropic gel forming properties under low temperature (for example, Patent document 3). However, any proposed methods result in insufficient image quality such as remaining surface stickiness and abrasion resistance being lower than expected, when using an ultra-violet irradiation apparatus such as a high-pressure mercury vapor lamp affects deformation on a recording medium such as shrink film which is weak for heat and stable printing cannot be available.
On the other hand, as a method for using general photo-curable type inkjet ink, an inkjet printer by ultra-violet curing method is proposed which irradiates high illuminance light after irradiation of low illuminance light (for example Patent Document 4).
Further as a method for printing on a recording medium such as shrink film which is weak for heat, an inkjet printer by an ultra-violet curing method is proposed which irradiates low illuminance light in a long period by arranging two UV light sources before and after carriage along the transporting direction (for example, Patent Document 5).
However, because that above proposed inkjet recording method is based on a general photo-curable inkjet recording method, an ejected ink deposits on the recording medium with low viscosity, and there exists time lag between deposition of ejected ink and light irradiation. Therefore, bleed occurred during this time lag cannot be prevented enough, and an improvement of the bleed is insufficient. Furthermore, in the case of applying a hot-melt photo-curable inkjet ink as it is as the general photo-curable inkjet recording method, it was found that prevention of bleed after curing, surface stickiness and abrasion resistance proved to be incompatible and insufficient by these methods. The cause is realized by a different curing behavior between through high viscosity stage by gelling and directly from low viscosity ink.    Patent Document 1: Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. 2005-126507    Patent Document 2: JP-A No. 2007-063553    Patent Document 3: WO2007/025893    Patent Document 4: JP-A No. 2008-100493    Patent Document 5: WO2007/058796